This invention relates to a black reference data calculation method of an image pick-up system used with an image reader and an image reader.
Generally, an image reader for applying light from a light source to an original placed on an original mounting stage and reading light reflected on the original or passing through the original by a image pick-up system as image data is known.
The image pick-up system used with the image reader as mentioned above has a plurality of pixels implemented as charge-coupled device. An electric signal responsive to the amount of received light is output from each of the pixels constructing the image pick-up system.
Since a plurality of pixels constructing the image pick-up system, the output value varies from one pixel to another to some extent. The light amount of a mercury lamp, a xenon lamp, etc., for example, used as a light source of the image reader in either end portion differs from the light amount in the center portion; both end portions become dark as compared with the center portion.
Thus, variations in output values of pixels or variations in light amounts of a light source is corrected by making a shading correction to the electric signal output from each pixel.
To make a shading correction, before the original read operation is executed, white reference and black reference are read and a correction value is calculated based on the electric signal output from each pixel. As for the white reference, for example, a white reference member of high reflectivity disposed on the rear of an original guide for defining the position of an original is read, whereby white reference correction data for each pixel is created. On the other hand, for the black reference, the output value of the electric signal output from each pixel of image pick-up system is detected with the light source off, whereby variations in the output values of the pixels are detected and black reference data is created.
The created black reference data is stored in a storage section of RAM, etc., and is used to correct the electric signal output from each pixel when an original is read. Arts for creating and storing the black reference data for each pixel of image pick-up system are disclosed in JP-A-4-25285 and JP-A-5-268474.
However, according to the arts disclosed in JP-A-4-25285 and JP-A-5-268474, the black reference data is stored in the storage section for each pixel and thus the storage section requires a capacity responsive to the number of pixels. Consequently, a large storage capacity is required for storing the black reference data.
According to the art disclosed in JP-A-5-268474, a method of detecting the electric signal output from each pixel about several times and setting the black reference data for creating black reference data with the decreased effect of random noise is disclosed. However, if the electric signal is detected about several times and the detection values are averaged, accurate black reference data with the decreased effect of random noise cannot be provided, resulting in a problem of increasing noise contained in an image.